During the production of cellulosic fibrous material pulp by continuous processes, such as the chemical or semi-chemical digestion of pulp in continuous digesters, or the production of mechanical pulp in refiners or the like, there are many circumstances in which it is desirable to change from a first raw material to a second raw material. For example it is very common to first be producing pulp from a coniferous wood (e.g. pine wood chips), and then switch over to the production of pulp from a deciduous wood (e.g. oak wood chips). If the production is stopped while changeover occurs so that there is no intermixing of the coniferous wood pulp and the deciduous wood pulp, the costs to the mill are substantial, therefore shutdowns are to be avoided. However during the changeover, a mixture of deciduous and coniferous pulps will be produced, which pulp mixture is not suitable for either of the end uses to which the coniferous or deciduous pulps are normally put. The pulp mixture may adversely influence the strength or other properties of the deciduous or coniferous pulps. The quantity of pulp mixture produced can be considerable in large production units. For example for a continuous digester that produces 1,000 tons of chemical pulp per day, the pulp mixture as a result of changeover of raw material can be about 40 to 50 tons.
According to the present invention it is possible to utilize the pulp mixture (e.g. the 40 to 50 tons in the example set forth above) in such a way that it will not be wasted, yet will not have an adverse affect on the properties of either the coniferous or deciduous wood pulps (for example). According to the present invention this is accomplished by terminating the feeding of the first pulp being produced by the production apparatus, and after changeover to the second material feeding the pulp mixture produced during the changeover to a screening station.
At the screening station, in the practice of the invention, the pulp mixture is separated into pulp fractions having properties corresponding to the first and second pulps, and the separated pulp fractions are fed to different locations for storage or treatment. Once the pulp mixture contains substantially all second pulp (that is after about 40-50 tons of pulp mixture has passed through, in the example set forth above), then the steps of feeding the pulp mixture to a screening station, and screening, are terminated, and all of the pulp is passed to a second treatment or storage vessel.
Typically the most significant different property of the first and second pulps is fiber length. The fiber length of the pulp mixture may be sensed prior to the screening station, and the proportion of pulp discharged from the screening station into either the first or second storage/treatment vessel is controlled in response to the fiber length analysis.
The screening station may be disposed directly in the discharge from the continuous treatment vessel (e.g. digester), or alternatively the pulp mixture can be fed to a storage vessel and fed from the storage vessel to the screening station. In this latter case, pumping and screening of the pulp mixture may take place over a longer period of time than it took to produce the pulp mixture.
The invention also comprises: A main vessel. At least three discharge conduits operatively connected to a discharge from the main vessel. Means for valving the flow of pulp from the main vessel so that it passes into one of the three discharge conduits. A first discharge conduit operatively connected to a first vessel. A second discharge conduit operatively connected to a pulp screen having an accepts discharge and rejects discharge. A third discharge conduit operatively connected to a second vessel. An analyzer for analyzing fiber length operatively connected to the conduits, and between the screen and the main vessel; and, the accepts discharge from the screen operatively connected to the first vessel, and the rejects discharge from the screen operatively connected to the second vessel.
It is particularly advantageous according to the invention that the continuous production of pulp, with minimum waste, can take place without diluting the pulp for conventional screening operations. By utilizing a screening apparatus that is capable of screening pulp having a consistency of between about 8-15% (the typical production consistency), it is possible to avoid the energy waste and the larger scale equipment that is necessary for diluting and then rethickening the pulp to screen it at low consistency.
It is the primary object of the present invention to provide an effective method for continuously producing cellulosic fiberous material pulp during changeover of a pulp production process from a first raw material to a second raw material so as to minimize the waste of the pulp while maintaining the integrity of the final pulp produced. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.